This invention relates to a method of detecting deterioration a three-way catalyst which is arranged across an exhaust pipe of an internal combustion engine for purifying exhaust gases emitted therefrom, and more particularly to a method of detecting deterioration of the three-way catalyst based on outputs from two air-fuel ratio sensors arranged in the exhaust pipe at locations upstream of the catalyst and downstream of same, respectively, at least one of which sensors is used for the air-fuel ratio feedback control.
Recently, from the viewpoint of reducing air pollution caused by exhaust gases emitted from internal combustion engines installed on automotive vehicles, more attention has come to be paid to deterioration of three-way catalysts provided in the engines for purifying the exhaust gases.
Conventionally, a method of detecting deterioration of a three-way catalyst is known from a paper SAE 900062 published by Society of Automotive Engineers in the United States of America, which is applied to an internal combustion engine including a three-way catalyst arranged in an exhaust pipe thereof, and a pre-catalyst oxygen sensor and a post-catalyst oxygen sensor (i.e. air-fuel ratio sensors respectively arranged in the exhaust pipe at locations upstream of the catalyst and downstream of same for detecting the air-fuel ratio of an air-fuel mixture supplied to the engine), and wherein deterioration of the three-way catalyst is detected by the steps of calculating areas defined between respective curves depicted by output values from the pre-catalyst and post-catalyst oxygen sensors and a predetermined reference line, calculating the difference between the two areas, and comparing the difference with a predetermined reference value.
However, according to the conventional method, no consideration is given to a difference in performance between the pre-catalyst and post-catalyst oxygen sensors, which is caused by variations between the sensors. Therefore, there can be a case where the difference in the two areas does not reach the predetermined reference value due to the difference in performance between the sensors in spite of the fact that the three-way catalyst is normally functioning, whereby it is erroneously determined that the three-way catalyst is deteriorated beyond a critical level.